Techniques for melting or softening small feed particles under controlled conditions to make generally ellipsoidal particulate products are known. Examples include atomization, fire polishing and direct fusion.
Atomization involves melting myriad feed particles to convert them to bulk liquid glass. A thin stream of such glass is atomized through contact with a disruptive air jet. It divides the stream into fine droplets. These are kept away from one another and from other objects until they cool and solidify. Then they are recovered as substantially discrete, generally ellipsoidal glassy, amorphous particles.
In fire-polishing, discrete, irregularly shaped glassy solid feed particles are heated to a soft or molten condition while dispersed and suspended in a hot gaseous medium. Surface tension forms the particles into ellipsoidal shapes. Kept suspended in cooler gases until reaching their freezing temperatures, the particles are recovered as solid, generally discrete glassy ellipsoids.
Atomization and fire polishing of glasses may be described as indirect methods. Their feed materials have been formulated from glass-making raw materials which were melted and homogenized in the form of bulk liquid prior to entering the ellipsoid-forming step. Consequently, in indirect methods, the individual chemical identities of the glass-making raw materials have been merged into an average composition which is uniformly present in the respective ellipsoids so produced.
Direct fusion, somewhat similar to fire-polishing, uses feed particles with irregular shapes that are not glassy, or are at least not fully glassy. They may be discrete solid particles and/or adherent groups of particles. These groups are sometimes referred to as clusters or agglomerates. Heated while in suspension and dispersion in a hot gaseous medium, the feed particles are softened or melted and formed into molten, generally ellipsoidal shapes, followed by cooling, freezing and recovery in an at least partly, but more fully, glassy state.
Unlike direct fusion, fire-polishing typically employs solid feed particles that are in a relatively highly or fully glassy or amorphous state. At some point in their history, they have existed in bulk liquid form. In direct fusion, feed particles that are not fully glassy or amorphous, and that are often non-glassy minerals, undergo direct conversion to glassy form, or at least to a more nearly glassy and amorphous form, in an ellipsoid-forming step, without prior conversion to bulk liquid form. Flame fusion, as employed herein, involves formation of at least partly fused, substantially glassy particulate products by direct fusion or fire-polishing of solid feed particles having physical states, as fed to a fusion zone, ranging from fully crystalline to fully glassy and amorphous.
In direct fusion, each ellipsoidal product particle may be formed by fusion of either a discrete feed particle or by fusion of a group of several mutually adherent feed particles, described herein as agglomerated. These product particles respectively and generally exhibit the varying chemical compositions of the discrete particles and/or the average chemical compositions of the groups of agglomerated particles, from which the ellipsoids are respectively formed, except that there may be relatively small losses of ingredients through high-temperature volatilization. Thus, direct fusion products do not necessarily have the more uniformly similar particle-to-particle composition expected of particles produced from bulk liquid glass by atomization or fire-polishing.
Various forms of equipment, as well as differing forms of feed handling and fusion methodology have been employed in known flame fusion processes. For example, as early as 1935, it was taught in U.S. Pat. No. 1,995,803 to Gilbert, at page 1, column 1, lines 31-32 and at column 2, lines 33-41, that in order to generate well-formed spherulized products, feed particles should be positively dispersed in the fuel and/or oxygen-containing gas that is fed to a burner that heats the fusion zone, and that this can be done upstream of the burner. Gilbert also teaches, at page 2, column 1, lines 1-8, that subsequent heating and expansion of these gases provides an additional dispersive effect. This patent does not disclose the geometry of Gilbert's combustion chamber. However, his later U.S. Pat. No. 2,044,680, at page 3, column 1, lines 2 and 5, twice describes his chamber as having "confining" surfaces.
As a further example, Garnier, in U.S. Pat. No. 4,778,502, at column 2, lines 41-45, discusses production of hollow microspheres from particulate feeds. At least 90 percent by weight of the feed particles have particle sizes less than 20, and preferably less than 10, microns. To combat agglomeration of the feed, which is recognized as making difficult the production of microspheres of small dimensions, the patent proposes pre-treating the feed by distributing over its particle surfaces a small amount of a "fluidizing agent," preferably alkanol amine(s). See column 2, lines 46-58. Feed, ball-milled with such agent, can be dispersed in gases, as taught at column 6, lines 19-35 and column 4, lines 50-55, and then fused with the aid of either of two burner types. Each of these, as described at column 4, line 64 through column 5, line 43 and in FIGS. 1 and 2, has a combustion chamber which is of restricted cross-section relative to a down-stream expansion enclosure. The combustion chamber, which includes fuel ports 20 and air ports 23,24, has an extension of equally restricted cross-section surfaced with refractory 25 (FIG. 1) or a liquid-cooled metal wall 27 (FIG. 2). In the FIG. 1 burner, the feed dispersion is projected into combustion gases departing the front end or outlet of the combustion chamber through one or more radially oriented injection ports 30,31. In the burner of FIG. 2, the feed dispersion is projected into the combustion chamber through an axial pipe 17 in the back end of the burner.
In British Patent No. 2,178,024, at page 5, line 33 through page 6, line 4, Mouligneau et al. say that it is most desirable to use feed well dispersed in the combustible gases. They teach propelling a stream of gas with entrained feed through a passageway leading to the combustion chamber and forcing a second stream of gas transversely into the first stream through an orifice in the passageway wall, to generate forces said to promote intimate admixture. Also, at page 2, lines 6-8, these patentees describe a tendency for feed particles to agglomerate and/or stick to the fusion chamber walls. They attributed this problem to excessive heating of the feed during fusion. As a solution, they proposed, at page 2, lines 15-20, to provide a flowing gaseous sleeve. It surrounded the stream of flaming combustible gases containing dispersed feed particles. The sleeve was said to improve yields of high quality beads by keeping the feed particles wholly enveloped in the flame, encouraging rapid heating of the feed, adding kinetic energy to the feed and product particles while keeping them dispersed and promoting rapid departure of product particles from the fusion chamber, cooling the fusion chamber walls and thus reducing agglomeration and sticking tendencies. See page 2, lines 22-31.
Morishita, et al., in Japanese published patent application HEI 2[1990]59416, published Feb. 28, 1990, discuss direct fusion of silica with particle sizes of less than 10 microns. Severe problems of agglomeration of the feed materials in the flame during fusion and adherence of particles to the furnace wall are mentioned. They suggest agglomeration may be prevented by working with plasma induction at temperatures exceeding those of the usual fusion furnace. However, they explain that this method is not suitable for mass production and has poor energy efficiency. Morishita, et al. proposed to solve these problems by using feed powder reduced by jet mill to less than 10 micron particle size, followed by direct fusion in a fusion furnace with an oxygenflammable gas (e.g., oxygen-propane) flame. Feed is supplied to a burner having a powder discharge port at the center, and an opening for the gas flame at the center axis. The thermal load of the burner and the thermal load per unit volume of the furnace were respectively in the ranges of 100,000-200,000 kcal/H and less than 2,000,000 kcal/m.sup.3 H. Higher thermal loads were said to lead to agglomeration of the feed, and lower burner thermal loads were said to lead to products of poor quality.
Commenting further on their above-described work, the above inventors and one other, in Japanese published patent application HEI 2[1990]199013, published Aug. 7, 1990, acknowledge that it proved difficult for them to make fine spheroidal silica at high yield by direct reduction of fine silica with control of thermal load. However, they suggest that this problem may be overcome by supplying a cooling gas to, and adjustment of, the flame generating area. Working with a fusion furnace with an oxygen-flammable gas flame again, and with less than 10 micron feed which is dispersed in carrier gas and fed to the center of the flame, they blow in cooling gas perpendicular to the flame or introduce it through a ring. This is done at a selected position downstream of the burner and is said to effectively eliminate flame generation, i.e., quench the flame. By changing the position and other aspects of introduction of the cooling/quenching gas, it is said that one can adjust the residence time of the silica in the flame, prevent growth of the grains by agglomeration in the flame and recover high yields of small particles.
In Japanese published patent application No. HEI 4[1992]-147923, "Manufacturing Method of Spherical Microparticles," by T. Koyama, et al., published May 21, 1992, the inventors suggest, apparently in the attempt to recover very small products, grinding the raw material to a particle size in the range of 0.1 to 1 micron. However, it appears that the fusion procedure used suffers from some considerable agglomeration of the molten or soft particles.
Notwithstanding the progress made by prior workers in the art, it appears that there is a need for, and an opportunity to provide, further improvements in the yield and energy efficiency of flame fusion processes aimed at producing very fine generally ellipsoidal particles. This appears especially true in relation to mass production of products, from feeds in the particle size ranges with 50th percentiles (average particle size) of up to about 25, up to about 20, up to about 15 and up to about 10 microns, or with 90th percentiles of up to about 60, up to about 40 and up to about 30 microns, by volume. In production of these products, increasing production rates have tended to produce agglomeration and ensuing particle size growth during fusion, while agglomeration has been avoided at the expense of energy efficiency.
The present invention seeks to fulfill the above-stated need. This goal has been fulfilled, and solution in part by development of the methods disclosed below.